At the present time, electronic timepieces, and particularly electronic timepieces having a digital time display, are in widespread use. There is an increasing demand for timepieces which provide a longer battery life, or for smaller timepieces which utilize a smaller size of battery as a power source. One method which has been proposed to accomplish this is to use a lithium battery, rather than the silver oxide type of battery which is generally employed in a conventional timepiece. A lithium battery has the advantage of providing a greater energy capacity than a silver oxide battery of similar size, and also has the important advantage that the rate of internal discharge of the battery while it is not being actually used is much lower than in the case of a silver oxide battery. Thus, the shelf life, or storage life, of a lithium battery is significantly longer than that of a conventional type of electronic timepiece battery.
However, a problem arises in using a lithium battery to directly power an electronic timepiece. This is due to the fact that the voltage provided by a single cell lithium battery is approximately double that of a silver oxide battery, i.e. of the order of 3 volts, as compared with about 1.5 V for a silver oxide battery. Since certain portions of the circuitry of an electronic timepiece which comprise metal oxide silicon field effect transistor elements (usually abbreviated to MOS FETs) can operate effectively with a supply voltage of 1.5 V or less, it is obviously wasteful of battery power to apply the full voltage of a lithium battery directly to such portions of the circuitry. These portions of the timepiece circuitry include the crystal controlled timebase oscillator circuit, the frequency divider circuit, and the timekeeping counter circuit. However it is a feature of such circuits that, if the timebase oscillator circuit (which consumes a substantial proportion of battery power, since the transistors associated with this circuit are switched at a relatively high frequency) is designed to operate at a certain supply voltage (for example, 1.5 V) at maximum efficiency, then it is generally necessary to momentarily supply a somewhat higher supply voltage to the timebase oscillator circuit in order to initiate oscillation. For this reason, a method has been proposed in the prior art for utilizing a lithium battery in an electronic timepiece, whereby a control pulse is produced by a portion of the circuitry for a predetermined time after a battery is installed in the timepiece. While this control pulse is being produced, the full output voltage of the lithium battery is supplied to the timebase oscillator circuit, so operation of the oscillator circuit is initiated. When the control pulse is terminated, after a time determined for example by a resistance-capacitance time constant, then a stepped-down supply voltage is supplied to portions of the timepiece circuitry including the timebase oscillator circuit. The latter stepped-down supply voltage is produced by a circuit which is actuated by a low-frequency signal produced from the frequency divider circuit of the timepiece, and which steps down the output voltage of the lithium battery. Changeover from supply of the full battery voltage to supply of the stepped-down voltage is accomplished by means of a changeover circuit, which is responsive to the control pulse referred to above, i.e. once the control pulse is terminated after the battery has been installed, the changeover circuit begins to supply the timebase oscillator and other circuits with a stepped-down supply voltage, rather than the full battery voltage. However, it is a feature of a lithium battery, as well as most other batteries, that a change in operating conditions, such as a lowering of the ambient operating temperature, will cause a reduction of the battery voltage. Thus, with a timepiece voltage control system such as that just described, it is possible that the battery voltage may fall, due to a change in ambient operating conditions, to such an extent that the stepped-down battery voltage level is insufficient to sustain operation of the timebase oscillator circuit. The oscillator circuit output therefore disappears, so that the voltage step-down circuit ceases to receive an output signal from the frequency divider circuit. In order to restore operation of the timepiece in such a case, it is necessary to remove the timepiece battery and then replace it, thereby again generating an initiating control pulse whereby the full battery voltage is supplied to the timebase oscillator circuit, thereby restarting operation of the oscillator circuit. Such a procedure is obviously troublesome, and in any case could not, in most cases, be performed by the timepiece user.
A modification of the above system has been proposed whereby an external operating member can be actuated to cause a changeover circuit to temporarily supply the full voltage of the lithium battery to the timebase oscillator circuit. This external operating member is actuated in order to initiate operation of the timebase oscillator circuit when a battery is first installed in the timepiece. Furthermore, if the battery voltage should subsequently fall to such an extent that the timebase oscillator ceases to function, due to a change in ambient operating conditions, then operation of the timebase oscillator can be restarted by the user actuating the external control member referred to. Such a system is obviously troublesome and inconvenient to the user. In addition, since it is necessary to incorporate an external operating member in the timepiece, the manufacturing cost will be substantially increased. It is a feature of an electronic timepiece that such components as electronic circuit elements can be added with a negligible increase in manufacturing costs, but that addition of any component such as an externally actuated switch results in a significant increase in cost.
In addition, of course, with either of the systems described above, there will usually be a relatively long time lapse between the cessation of operation of the timebase oscillator circuit and restoration of operation of the timepiece. It will therefore be necessary for the user to reset the current time displayed by the timepiece after operation has been restored. Such systems are therefore inherently undesirable.
With the present invention, the disadvantages of such prior art systems are effectively eliminated. In an electronic timepiece powered by a lithium battery, cessation of operation of the timebase oscillator circuit while a stepped-down supply voltage is being applied to that circuit, is immediately detected, and the full voltage of the lithium battery is supplied to the timebase oscillator circuit. Operation of the oscillator is thereby immediately restored. In addition, the voltage control system of the present invention automatically supplies the full voltage of the lithium battery to the timebase oscillator circuit at the time of initial installation of a battery, and changes over to supply of a stepped-down voltage to the oscillator circuit as soon as operation of that circuit has commenced. No action by the timepiece user, or by the person who instals the battery, is required. A voltage control system according to the present invention therefore offers significant advantages in terms of user convenience, as compared with prior art systems proposed for utilizing a lithium battery in an electronic timepiece. In addition, since no external operating member is required, the use of a voltage control system according to the present invention involves no penalty in terms of increased manufacturing costs.